Breaking Down AR Upper Options

The good ol’ AR-15 is one of the most modular firearm platforms to have ever existed. It’s a lot like Barbie. It can be everything! The AR-15 platform can vary wildly in how it’s used based on how it’s designed, and one of the biggest ways to alter the purpose of the AR-15 is to swap uppers. The upper receiver makes a huge difference in how the weapon handles and how it performs. With that in mind, what are your more common AR-15 upper options? What difference do they make? We will explore both today. 

AR Rifle Uppers

Rifle-length AR uppers traditionally use a 20-inch barrel. The original AR-15 and later the original M16 would utilize a 20-inch barrel and what would become known as the rifle-length gas system. In the modern era, 18 to 20-inch barrels are considered rifle uppers. I believe that rifle upper receivers are slept on by the AR community. 

The benefits of an 18 to 20-inch barrel and a rifle-length gas system are aplenty. The longer gas system creates a smoother feeling of operation with less recoil. The longer gas system allows for a smoother experience. Additionally, the action is less violent and creates less wear on parts. However, this is a very minor benefit because the differences in rates of wear are relatively minor. 

FN M16A4 clone
The 20-inch barrel was what the AR-15 began its life with.

Rifle-length uppers also increase the velocity of the rounds fired. This helps maximize their terminal performance at range and increases their effective range. In fact, firing an M193 through a 20-inch barrel results in a round capable of piercing right through AR500 steel armor. The 20-inch barrel adds some real speed to the rounds fired, and that can be a significant benefit for piercing hard barriers. 

The downsides come down to the fact the barrel is 20 to 18 inches. This makes the gun less maneuverable for many people. However, the Marines cleared Fallujah with M16A4s with 20-inch barrels and did a great job in those small, Middle Eastern buildings. 

Designated Marksman Rifle 

The Designated Marksman Rifle, or DMR, upper is designed to maximize accuracy. Barrel length can vary between 18 to 22 inches. The Mk12 DMR rifle used by the United States military used an 18-inch barrel. As you’d expect, these use rifle-length gas systems and tend to be very smooth shooting designs. Off the bat, they do sound identical to a rifle-length system, and they are in many ways. They get the benefits of lower recoil, less violent action, and higher velocities. 

Daniel Defense DMR
The DMR uppers are accurized. All DMRs are rifles, but not all rifles are DMRs.

Where the DMR and rifle-length uppers differ is in their purpose-built nature. The DMR upper will be built to enhance accuracy in most ways. This includes a free-floating handguard and a stainless steel barrel, and the upper practicality has to be a flat top design to accommodate optics. The handguard is typically modular, so accessories like bipods are easily attached. The bore and rifles may be attuned to fire-specific weight cartridges to maximize their accuracy as well. 

DMR uppers may be specifically designed to be more rigid. Barrels can also be heavier and more rigid to accommodate more accurate fire and to provide longer strings of accurate fire. The DMR uppers are typically more expensive and use top-tier pieces to ensure the rifle is as accurate as possible. 


The Dissipator upper receivers are somewhat rare these days. They came to be during the Vietnam War. Colt produced the Model 605, cutting the barrel at the gas block and creating a 15 or so-inch barrel carbine that was quite a bit smaller and more maneuverable for jungle warfare. Sadly, it had some reliability issues and was canned by Colt. Years later, they solved the issues, and it’s now back in proper working order. 

Anderson Dissipator
The Dissipator is an underrated AR variant.

Moder Dissipators have 16-inch barrels that are paired with rifle-length gas systems. Several companies do make dissipator look-alikes with mid or carbine-length gas systems. Currently, it seems like only DelTon and Anderson produce actual Dissipators with rifle-length gas systems. Dissipators offer less recoil and a smoother operation than other carbines. With iron sights, they also offer a longer sight radius than standard carbines. 

Dissipator upper receivers work these days by using a wider gas port. This has solved the reliability issues and ensured the carbine cycles reliability. Dissipators often wear longer, rifle-length handguards, which offers more room for accessories. However, they all have a fixed sight base, and the handguards are not free-floated. While a Dissipator upper with a low profile gas block and free float handguard is possible, they are not common. 


Carbine uppers are, without a doubt, the most popular AR-15 uppers on the planet. The traditional carbine used by the average shooter features a 16-inch barrel, but the actual length of a carbine barrel varies from 13.7 to 16 inches. Anything less than 16 inches requires the weapon to be a pistol, be an SBR, or have a permanently affixed muzzle device. 

bcm carbine
The Carbine is the most basic AR-15 Upper

These platforms are typically considered do-it-all profiles. They are long enough to generate substantial velocity for the average 300-yard engagement but short enough to easily use indoors when necessary. They are just right in size and Goldilocks-approved. Caribbean gas systems are the most popular length, but the midlength variety is becoming more popular and does offer a smoother recoil impulse. 

While the muzzle flash and concussion are increased from a rifle or DMR length barrel, it’s not a substantial increase by any means. Due to the popularity of this barrel length, there are tons of options out there with practically any handguard, upper, and barrel profile combination. 

Close Quarters Upper 

What is a close-quarters upper? To me, it’s anything from the Mk18 style 10.3 to the popular 12.5-inch upper. These platforms are designed for close-quarter use. The barrels are short to ensure the rifles are short, making them easy to use indoors, in and out of vehicles, and even to create a fairly compact rifle even when it’s equipped with a suppressor. The close-quarters upper receivers are either SBRs or AR pistols. 

Daniel Defense MK18
The MK18 was the first modern, super short carbine.

These uppers will typically feature a short handguard and be quite lightweight overall. The close-quarters uppers can be any number of calibers, including 5.56 and .300 Blackout, but we can also see 9mm, 5.7x28mm, and many more. In the most common AR round, 5.56, the shorter barrel does limit the velocity and effective range of the carbine.

It also creates harsher recoil and more extreme concussion and sound from the short barrel. The close-quarters upper is right around the limit for the effectiveness and reliability of the 5.56 caliber round. 


PDW, or personal defense weapon, is a term typically applied to ultra-small and compact platforms. In the AR world, I’d argue anything smaller than 10.3 inches falls into the PDW caliber realm. These are ultra-small upper receivers with short barrels and short handguards. They are used for AR pistols or short-barreled rifles. While plenty exists in 5.56, they are not best used in 5.56. 

Noveske ghetto blaster
The PDW, like the Ghetto Blaster, are uber-small platforms

The 5.56 tends to prefer longer barrels, and there have been reliability issues with ultra-short barrels and the 5.56. Another issue is that it creates quite the fireball and concussion in these super short barrels. It’s more of a noisemaker than an effective weapon. 

The PDW calibers work best with cartridges like .300 Blackout, 4.6x30mm, and 5.7×28, as well as pistol rounds like 9mm, 10mm, and similar. Personal defense weapon-length barrels are designed for short-range engagements and are ultra-small for close-quarters use. 

The Upper 

Your upper says a lot about what the rifle is used for. Upper receivers make a massive difference, and you can fairly easily swap between calibers by just swapping the upper. Sure, you might need to change a buffer here and there, but the upper is the main difference. I’m a fan of rifle upper receivers, but what’s your favorite? Let us know below! 

Travis Pike is a former Marine Machine Gunner and a lifelong firearms enthusiast. Now that his days of working a 240B like Charlie Parker on the sax are over he's a regular guy who likes to shoot, write, and find ways to combine the two. He holds an NRA certification as a Basic Pistol Instructor and is probably most likely the world's Okayest firearm instructor. He is a simplicisist when it comes to talking about himself in the 3rd person and a self-professed tactical hipster. Hit him up on Instagram, @travis.l.pike, with story ideas.

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